1. Field of the Invention
The invention relates to an apparatus and a method for producing a silicon single crystal by crystal pulling by the Czochralski method (the CZ method), and to a silicon single crystal produced using the same method.
2. Description of Related Art
To produce a silicon single crystal, polycrystalline silicon raw material is held in a crucible and is heated by a heater to form silicon melt, and the silicon single crystal is grown while being pulled up from of the silicon melt by the CZ method. Silicon wafers are produced by slicing (cutting) the silicon single crystal produced by the above described method, and integrated circuits are formed upon the silicon wafers. For forming integrated circuits upon a silicon wafer, there are demands for a silicon wafer with high quality and free from defects such as OSFs (Oxidation Induced Stacking Faults), COPs (Crystal-Originated Particles), L/DLs (Large Dislocation Loops), and the like.
Here, the OSFs are a type of crystal defect formed on the surface or in the interior of the silicon wafer and are stacking faults caused by oxidation. An OSF upon the surface of the silicon wafer is generated due to residual strain or contamination of a surface neighborhood, while an OSF interior to the silicon wafer is generated due to precipitates caused by supersaturated oxygen. The COPs are those defects of crystalline origin which occur at the surface of the silicon wafer, due to etching of the silicon single crystal during the washing process, and are counted as particles by a particle counter. The L/DLs are termed dislocation clusters, or dislocation pits, since, when a silicon wafer generating these defects is dipped into a selective etching liquid mainly composed of hydrofluoric acid, the defects generate oriented etch pits. The L/DLs also cause a deterioration of the electrical properties, such as leak properties, the isolation properties.
For a effective production of a silicon single crystal having a defect free region throughout the entire length, it is needed to maintain specific growth conditions of the silicon single crystal. For example, the following Patent References 1 to 5 disclose techniques for producing a silicon single crystal having a defect free region by pulling the silicon single crystal while applying a cusp magnetic field. In particular, in Patent References 1 and 4, a shape of the solid-liquid interface between the growing silicon single crystal and the silicon melt is made in an upwardly protruding shape in order to enlarge the defect free region, and practical controlling conditions to obtain this shape are disclosed. If the shape of the solid-liquid interface is controlled upwardly protruding, a thermal gradient is increased in the vertical direction at the center of the silicon single crystal during pulling, and as a result it becomes easy to obtain a defect free region, since the distribution of defects is made constant in the radial direction of the silicon single crystal (in other words, over the surface of a silicon wafer which sliced from the silicon single crystal).
According to Voronkov's theory (V. V. Voronkov) well known in the related field, where a value showing the ratio of the pulling speed of the silicon single crystal V and the thermal gradient G in the vertical direction within the silicon single crystal near the solid-liquid interface is given by V/G, according to the difference in V/G value whether exceeding or not exceeding a critical value, the silicon single crystal being produced has vacancy type point defects, or interstitial silicon type point defects. A defect free region is formed when the V/G value is near the critical value. If the V/G value cannot be controlled to be at or vicinity of abovementioned critical or near critical value throughout the entire extent of the silicon single crystal from its center to its circumference, a silicon wafer sliced from the produced silicon single crystal is a mixture of defect free region and defected bearing region. Accordingly, to obtain a defect free region, it is necessary to control the radial variation of the V/G to be low sufficiently. Furthermore, when the thermal gradient G in the vertical direction of the silicon single crystal is increased, to maintain the constant V/G value, pulling speed V must be increased resulting in the enhancement of a productivity of the silicon single crystal.
Since convection of the silicon melt have a great influence on the shape of the solid-liquid interface, in the following Patent References 1 to 5, a cusp magnetic field or a horizontal magnetic field is applied to the silicon melt, and by restricting magnetic field intensity and applying location of the magnetic field, further controlling the rotation speed of the silicon single crystal and the rotation speed of the crucible, convection of the silicon melt is controlled for setting the requisite shape of the solid-liquid interface.
To form an upwardly protruding shape of the solid-liquid interface as in Patent References 1 and 4, it is extremely important to control the up stream rising from the bottom of the crucible to the silicon melt surface to force up the liquid-solid interface to have an upwardly protruding shape.    Patent Reference 1: Japanese Unexamined Patent Application, First Publication No. 2001-158690.    Patent Reference 2: Japanese Unexamined Patent Application, First Publication No. 2000-272992.    Patent Reference 3: Japanese Unexamined Patent Application, First Publication No. 2003-002783.    Patent Reference 4: Japanese Unexamined Patent Application, First Publication No. 2003-002784.    Patent Reference 5: Japanese Unexamined Patent Application, First Publication No. 2003-055092.
As described above, a defect free region is formed within the silicon single crystal when the V/G value is a certain specified value. Furthermore, if it is possible to set the V/G value to be the specified value throughout the entire plane of the silicon single crystal, and maintain the V/G value at the specified value from starting to ending of pulling the silicon single crystal, a silicon single crystal having a defect free region throughout its entire length cane be produced.
However, since the allowable range for the V/G value capable of forming a defect free region is extremely narrow, it is extremely difficult to setting the V/G value to be the specified value throughout the entire plane of the silicon single crystal, and maintain the V/G value within the allowable range from starting to ending of pulling the silicon single crystal. Furthermore, it is also difficult to obtain very large thermal gradient G in the vertical direction while pulling the silicon single crystal. Therefore, increase of pulling speed of the silicon single crystal is restricted, thus being impossible to highly enhance the productivity of the silicon single crystal. Furthermore, the narrow allowable range of the V/G value results in narrow allowable range of the pulling speed of the silicon single crystal. By deviation of pulling speed from the allowable range, defects are generated partially in the silicon single crystal.
The objective of the invention being conceived in the light of the above described situation is to provide an apparatus and a method for producing a silicon single crystal effectively producing a silicon single crystal having a defect free region by relaxing various conditions for producing a single crystal having a defect free region, and a silicon single crystal produced using the method.